Although the sheet separating art is quite old, modern electrostatic copiers and electronic scanners have required its perfection. The most critical sheet separating task in a copier is the document handler. In order to maintain the page sequential order of a stack of original document sheets, the sheets are commonly fed off the bottom of the stack. Because of the weight of the stack, bottom separation is much more difficult than top separation. To further complicate the task, the copier is asked to handle used originals which vary in size, weight, age and condition. Not surprisingly, nearly all document handlers presently on the market come with instructions not to feed originals of unusual weight or of porr condition.
The predominant technology in high speed document handling involves separation of the bottom sheet from the stack with either a vacuum roller or a vacuum belt. These devices permit high speed feeding of a variety of documents without damage. However they generate noise which must be muffled, are expensive and require substantial power.
Scuff separating devices are much less expensive than vacuum devices, use less power and create less noise. They are commonly used in copiers and printers to feed blank copy sheets from the top of a stack. However, they have been considered too unreliable for bottom separating from a stack of used document sheets of varying characteristics. Prevention of double feeding and non-feeding in known bottom scuff feeders is difficult with such document sheets. More important, because of reliance upon friction and a certain amount of slippage, they have been considered likely to damage more frail originals.
U.S. Pat. Nos. 2,665,906 and 4,480,827 disclose top scuff separating devices in which the problems of slippage have been reduced by use of a braked retard roller opposite a larger drive roller. Although these structures have been in the literature for many years the principles have not been applied to the demands of bottom separating document handlers.